The fuel that is used in aircrafts has a density that changes with brand, additives, temperature and pressure, as well as other factors. Because the density of aircraft fuel changes in response to these factors, knowing the volume of fuel aboard an aircraft doesn't always sufficiently inform the pilot of the amount of fuel, and therefore the possible range of flight of the aircraft. The product of fuel volume and fuel density gives the fuel mass, which is a better metric of the amount of fuel carried by the aircraft. Many modern aircrafts are equipped with both fuel volume sensors and fuel density sensors.
Fluid density sensors or fluid densitometers are used to measure the density of a fluid. Such sensors can be constructed to exploit different principles of operation. For example, one of the oldest ways to measure density is by measuring a pressure differential between two bubble tubes vertically oriented in a fluid tank. In this method, the two bubble tubes are immersed in the fluid tank. Each of the bubble tubes extends to a different depth in the fluid tank. Air is pumped through each of the tubes so as to purge the tube of fluid, which results in bubbles being injected at the depth of the bubble tube. The back pressure is measured for each of the bubble tubes. The difference between the back pressures of the two bubble tubes is related to the density of the fluid in the tank.
Other methods of measuring a density of a fluid employ various mechanical oscillation measurements. For example, a spring may oscillate at a first resonant frequency when oscillating in air. But when submersed in a liquid, the spring may oscillate at a second resonant frequency. The resonant frequency of oscillation may be indicative of a density of the fluid in which the spring resides. Other electromechanical oscillators that vibrate at a frequency proportional to the density of the medium surrounding the oscillator can also be used to measure fluid density. A vibrating spool densitometer is one such fluid density transducer. These vibrating spool densitometers traditionally have electrical interfaces, with which to communicate operating power and output signals to a control unit.
Electrically communicating between a control unit outside of a fuel tank and an electromechanical oscillator within a fuel tank can incur risks of electrical arcing, which in turn can ignite the fuel within the tank. A short circuit or a lighting strike involving the electrical lines that connect the fuel density sensor to the controller can have catastrophic consequences. Thus, there is a need for eliminating electrical communications between equipment located within a fuel tank and controllers located outside the fuel tank.